The patent document US2005/0229981A1 discloses a tap for a pressurized gas cylinder comprising a valve with a control wheel and a pressure gauge mounted in the wheel and the control shaft of the valve. The pressure gauge is hydraulically downstream of the valve seat. It essentially comprises a sliding member with a piston housed in a cylinder, and a visual rotary member coupled to the sliding member. One face of the piston is subjected to fluid pressure downstream of the valve. The opposite face is subject to the resilient force of a spring. A rod extending from the piston comprises helical tracks cooperating with a corresponding orifice of the rotary element display, so as to convert the sliding movement of the piston and the rod into a rotary movement of the visual rotary member. The sliding member can take an infinite number of positions along the sliding stroke combined with defined angular positions, the latter corresponding to the angular positions of the visual rotary member. This indicating device is in fact a pressure gauge aiming to replace conventional gauges including shows gauges. The latter are indeed subject to deterioration when subject to impacts, especially when the cylinder falls down, this deterioration can be a source of leakage and hence a source of danger especially for gas under very high pressure and of dangerous nature. The pressure gauge of this teaching is also presented as being able to withstand pressures that are a multiple of its normal range of pressure. This pressure gauge is however of a somewhat complex design in particular in the mechanical connection between the rod and the visual rotary member as well as at the level of the guiding of the visual rotary member. Indeed, it must be supported so as to limit the friction forces, and more particularly in order to avoid blockages. Moreover, a blocking of the visual rotary member can cause visual rotation of the piston which does not appear to be linked in rotation with the cylinder in which it slides. This situation can generate an additional perturbation of the indicator as well its maladjustment. The location of the pressure gauge downstream of the shut-off valve is intended to permit verification of the state of pressure between the shut-off valve and the consumer connected to the tap, especially when it is desired to disconnect the consumer form the tap. Indeed, an improper valve shut-off at such a disconnect operation could have major consequences in terms of security. Unreliable indicator can, in these circumstances, constitute a substantial impairment of a security point of view.
The patent document GB 888,182 A discloses a pressure gauge for screwing through a wall of a pipe, tank or other. The pressure gauge operates on a similar principle to the previous document. However, it differs essentially in that the piston is not sealingly sliding in a cylinder, but actually comprises a corrugated flexible wall serving as sealing means and also of elastic means. It also differs in that the visual rotary element is supported by a ball bearing so as to ensure smooth operation without locking the indicator. This construction is therefore interesting from a reliability point of view however has disadvantages of implementation costs and bulk.